Non-invasive imaging technologies allow images of the internal structures or features of a patient/object to be obtained without performing an invasive procedure on the patient/object. In particular, such non-invasive imaging technologies rely on various physical principles (such as the differential transmission of X-rays through a target volume, the reflection of acoustic waves within the volume, the paramagnetic properties of different tissues and materials within the volume, the breakdown of targeted radionuclides within the body, and so forth) to acquire data and to construct images or otherwise represent the observed internal features of the patient/object.
In the case of cardiac scans, the motion associated with the heart and respiration may provide challenges to acquiring useful images. To help account for such motion, cardiac scans typically require the use of an electrocardiograph (ECG) device which measures electrical impulses traveling across the heart muscle to trigger contractions. The ECG can monitor cardiac phase within a cardiac cycle such as diastole and systole from the signal. Typically, least cardiac motion is observed in end-diastole and end-systole phases of the cardiac cycle.
In traditional cardiac CT scanning, the ECG signal is recorded during the CT scan acquisition to trigger the CT scan during a quiescent phase and minimize cardiac motion artifacts or to retrospectively select the time interval at which to center the reconstruction (relative to the entire scan duration over which data was collected). Typically, some “phase padding” is included, i.e. the scan duration is extended over some extra time window to make sure at least some portion of the scan happened at a quiescent phase. In ECG modulation the ECG signal is used to modulate the tube current, i.e., to increase it, during the desired cardiac phase. However, recording an ECG signal typically requires placing a multi-lead ECG apparatus, which is time-consuming and prone to errors.
For some CT applications such as CT angiography, a contrast agent (e.g. iodine) is injected through a vein to increase visibility of blood vessels and other organs. Use of analogous contrast enhancing agents may be employed in other imaging modalities as well, such as in the context of magnetic resonance imaging (MRI). Since it takes multiple seconds for the contrast bolus to arrive in the organs of interest and the bolus disappears again a few seconds later, it is important to time the CT scan appropriately so that images are acquired when the contrast agent concentration is at (or near) peak in the target region of interest (ROI) or vessel, or generally near the desired phase of the contrast uptake and/or washout process.
Traditionally, during a monitoring phase scanning at low dose and real-time image reconstruction is repeatedly performed to track the contrast bolus level of a target vessel (or appropriately selected nearby anatomy). Once the bolus level exceeds a threshold in the reconstructed image, the diagnostic CT scan starts. However, the extra scan and reconstruction steps are time consuming and need to be done repeatedly. Moreover, there is always some degree of delay between the preparatory or monitoring phase and the actual diagnostic phase, so the prediction of when the right level of opacification will occur may be inaccurate.